UV-curable adhesive semiconductor chip mounting process

ABSTRACT

Assembly of semiconductor chips is accomplished using an acrylate-based adhesive which can be cured rapidly by UV light and which gives an adhesive bond that is resistant to soldering by can be readily removed by solvents after soldering. During assembly, the back sides of the semiconductor chips are bonded to a temporary carrier by the adhesive and adjusted to the proper mirror image position on the carrier before the adhesive is cured. The chip array is then soldered on a component and the temporary carrier is removed from the chips by a solvent.

This is a continuation of application Ser. No. 07/809,482, filed Feb.21, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a UV-curable adhesive composition for asemiconductor chip mounting process and also to said mounting process.

2. Description of the Related Art

In the course of the development of semiconductor devices, theperformance of said devices is being increased by progressiveminiaturization of the devices together with the structures containedthereon. If technical reasons require a certain spatial size for adevice which exceeds that of an individual device, a large device isoften assembled in modular construction from smaller units. Depending onthe complexity of the circuits or the size of the structures containedthereon, a spatially precise relative arrangement of the individualmodules with respect to one another or to the substrate or carrier maybe necessary in assembling the individual modules, which may be similaror even different. The maximum permissible error tolerances may in thatcase be in the region of a few μm and are often even less.

WO 89/08896, for example, describes a nonmechanical printer functioningon the electrophotographic principle. Its character generator contains amultiplicity of light sources which are arranged in an exposure line.These are normally light-emitting diodes (LEDs) which are, for example,integrated in two rows of 64 LEDs each on a monolithic chip. The chipsare in turn assembled on individual modules from which the exposure linecan in turn be assembled in accordance with the paper format to beprinted. The achievable printing quality depends on the fineness of thescreen, that is to say on the spacing of the individual LEDs on thechip. High-resolution character generators currently achieve a printscreen of 600 dots per inch (dpi). With an exposure line 450 mm wide,this corresponds, for example, to a quantity of over 10,000 individualLEDs which are adjacently arranged in two rows with gap offset. In ordernot to adversely affect the printing quality at this printing density,it is necessary to maintain an error tolerance of ±2 μm when assemblingthe individual chips on a module. For this purpose, the individual chipshave to be precisely aligned during mounting on the module before thefinal fixing. After the alignment, the position of the chips must nolonger change during the fixing. For the soldering process normally usedfor this purpose, this requires, however, extensive precautions since,as is known, the solder fuses during soldering but sliding of the chips,in particular, should be avoided.

Although chips can be fixed with positional accuracy by gluing, a highheat loss has to be dissipated during operation because of the highpower density of the integrated chips, and this is only possible via asolder joint.

German Published Application 38 08 667 proposes gluing the chips onto atemporary carrier in an arrangement which is mirror-inverted withrespect to the finished module, the precise alignment already beingcarried out. In this fixed form, the chips are then soldered onto theactual module. After completion of the soldering process, the adhesivebond is released and the temporary carrier consequently removed.However, it has hitherto not been possible to find for this process asuitable adhesive which both makes possible a rapid fixing of the chipson the temporary carrier, the fixing being resistant in the solderingprocess, and can also be removed again completely after the solderingprocess.

From IBM Technical Disclosure Bulletin 13, (1971), page 3652, it isknown to glue a semiconductor wafer to a temporary carrier with the aidof a photoresist, to divide the wafer into individual chips, to fix thebonding of serviceable chips by means of UV light and, finally, torelease the fixed bonds again with the aid of a solvent.

EP-0 142 783 discloses a process for mounting semiconductor devices on asubstrate, in which the devices are first glued to the substrate withthe aid of a thermally curable adhesive containing styrene polymers.After thermal curing, the devices are soldered on and, finally, theadhesive is removed again with the aid of a solvent.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide anadhesive which can be cured with UV light and which makes possible arapid and reliable fixing of the chips, for example, on the temporarycarrier, which is resistant to the soldering process and which cansubsequently be removed without leaving a residue.

This object is achieved by a UV-curable adhesive composition whichcontains, according to the invention,

A) at least one monofunctional (meth)acrylate of an aliphatic orcycloaliphatic alcohol containing 6-20 carbon atoms,

B) a polyfunctional reactive crosslinking agent based on alow-molecular-weight aliphatic polyhydric alcohol esterified with(meth)acrylic acid,

C) oligomers or polymers containing olefinically unsaturated groups,which can be copolymerized with acrylates and have a slight crosslinkingaction, for the purpose of flexibilization,

D) a photoinitiator system for UV light, and also

E) a thermally activable radical starter, from which adhesivecomposition an adhesive bond can be produced which is resistant to asoldering process and which can also be dissolved with a solvent afterthe soldering process to leave no residue.

Further developments of the invention and also a process for mountingsemiconductor chips on a device are provided by the present invention,wherein the adhesive composition contains further standard additivessuch as coinitiators, stabilizers or pigments. More specifically, theadhesive composition provides that the component A is present in aproportion of 30 to 70 percent by weight. In addition, the component Cis preferably a butadiene/acrylonitrile copolymer containing vinylterminal groups present in a proportion of 10 to 50 percent by weight. Aperoxide is present as the radical starter. Preferably, a suitable amineis present which eliminates the oxygen inhibition during the UV curingof the adhesive composition. The adhesive composition contains aphotoinitiator system which is predominantly sensitive to light below400 nm.

A process for mounting semiconductor chips on a device with the aid ofan adhesive composition as disclosed above, comprises the followingprocess steps:

application of a UV-curable adhesive layer to the regions of a temporarycarrier provided as adhesion sites,

placing of the semiconductor chips to be mounted over the adhesion siteson the temporary carrier with the active side opposite the latersoldering site in a mirror-inverted arrangement (with respect to thelater mounting on the device),

determining the precise positions of the semiconductor chips on thetemporary carrier relative to a specified position and, if necessary,aligning the semiconductor chips in accordance with said specifiedpositions,

fixing the positions of the semiconductor chips by irradiating theadhesive layer with UV light,

curing the adhesive layer at elevated temperature,

soldering the rear side of the semiconductor chips glued to thetemporary carrier to the device (5) in a soldering process, and

releasing the adhesive bond between the semiconductor chips and thetemporary carrier and dissolving all the adhesive residues on thesemiconductor chips by means of a solvent.

The process preferably further includes the relative positions of thesemiconductor chips on the carrier being determined by means of opticalmeasuring methods, in particular by means of a microscope, and theshorter-wave components being filtered out of the light used for thispurpose by means of a color filter. The light used to fix the adhesivebond is preferably presented by means of an optical waveguide.Vapor-phase soldering is carried out as the soldering process. Aphotoresist stripper is preferably used as the solvent.

The adhesive composition according to the invention is a material whichcures rapidly when exposed to UV irradiation. This is advantageous, inparticular, when a multiplicity of semiconductor devices are to bemounted on a carrier or module, the devices advantageously beingindividually aligned and immediately fixed. The time necessary to fixall the semiconductor devices on the carrier or module is then theproduct of the curing time for an adhesive bond and the number of thedevices to be mounted on the module. The curing time of the adhesivecompositions according to the invention which is in the region ofseconds is therefore particularly advantageous since it requires onlyshort machine utilization times and makes possible economical mounting.

The adhesive bonds already immovably fixed by the UV curing arecompletely cured in a thermal process. This is brought about by thethermally activable radical starters contained in the adhesivecomposition. A multiplicity of compounds suitable for this purposesmakes it possible to adjust the curing to a desired temperature by meansof a choice which depends, for example, on the decomposition temperatureof the radical starters.

The composition according to the invention is a balanced mixture ofmonofunctional and polyfunctional constituents which result in a resinmatrix which is also sufficiently structurally stable thermally but atthe same time contains polymers which are crosslinked to a sufficientlysmall extent to ensure a problem-free dissolving of the cured adhesivecompositions in organic solvents. A completely new property profile ofthe adhesive composition, which combines objectives, contradictory perse, in an unexpected and nevertheless advantageous manner, is thusachieved.

For the UV sensitivity, a photoinitiator system is provided which yieldsfurther advantages compared with a single photoinitiator which is alsopossible in principle. This is, in particular, a higher sensitivity inrelation to the intensity and the wavelength range covered of the lightused for curing. In this connection, the sensitivity of thephotoinitiator system and, consequently, of the entire adhesivecomposition is so dimensioned that the latter can be processed withartificial illumination without an undesirable premature curing and theincrease in the viscosity of the adhesive composition associatedtherewith being brought about at the same time. In addition, the preciseposition of the semiconductor devices on the module is determined beforefixing in the normal way with optical methods which, if the longwavecharacteristic of the radiation used for the measurement is appropriate,also does not bring about premature curing of the adhesive composition.Incidentally, the latter can be stamped in layer thicknesses of, forexample, 5 μm and exhibits good adhesion to glass, metal and chipsurfaces which are the carrier materials or surfaces preferably used foran LED line in the specified mounting process.

The optional addition of a pigment dyestuff to the normally colorlessadhesive composition makes possible a visual check of the adhesiveapplied or its removal by solvents.

Coinitiators are capable of accelerating the curing, for example byeliminating oxygen inhibition during the curing process. Generally, thepresence of oxygen in a light-cured mixture brings about a decrease inthe reaction rate which can even result in inhibition, particularly inthe regions near the surface. The oxygen inhibition can be reduced bysuitable additives such as are also contained in the adhesivecomposition according to the invention, so that it is possible toprocess the adhesive composition even in a normal atmosphere. This cantherefore be done by a machine or by hand.

The low-molecular-weight and monofunctional acrylates or methacrylatesselected are preferably those of aliphatic or cycloaliphatic alcoholscontaining 6 to 20 carbon atoms. They are present in the composition ina proportion of 30 to 70 percent by weight. The monofunctional compoundsare the more readily soluble components of an adhesive bond composed ofthe cured adhesive composition. As homopolymers, they form linear anduncrosslinked polymers and are therefore readily soluble. In theadhesive composition, they form the main constituent and theirproperties therefore also predominantly determine the properties of theadhesive composition or of a cured adhesive bond. Acrylates of aliphaticbicyclic alcohols have rigid molecular structures, cure with lowshrinkage and, as homopolymers, have a high second order transitiontemperature which, with an increasing proportion in the adhesivecomposition, can also be transmitted to the latter. Furthermonofunctional acrylates can be derived from linear aliphatic alcohols.Their relatively long and readily mobile aliphatic radicals make them acomponent which reduces the viscosity of an adhesive bond. Ashomopolymers, they yield gel-like to rubber-like polymers with goodsolubility and low second order transition temperature. As monomers,they are low viscosity and dilute the adhesive composition so that aviscosity of the adhesive composition suitable for the processing can beestablished by means of their proportion. To use the adhesivecomposition in the stamp application technique a viscosity of, forexample, approximately 600 mPas or less is suitable.

As a so-called crosslinking intensifier, the adhesive compositioncontains a polyfunctional acrylate, preferably a low-molecular-weightaliphatic polyhydric alcohol esterified with acrylic acid or methacrylicacid. During the curing process, it ensures a crosslinking of theadhesive composition and, in this process, increases the mechanicalhardness and strength, the solubility of the cured adhesive compositionbeing reduced at the same time. Advantageously, it has three or morefunctional groups and can therefore be derived, for example, frompentaerythritol, trimethylolpropane or similar compounds. The proportionof the crosslinking intensifier in the adhesive composition is criticalin relation to the properties of the cured adhesive bond and, dependingon the polymer system used, is between 3 and 15 percent by weight.

As component C) the adhesive composition according to the inventioncontains an oligomer or polymer which is essentially linear and containsa plurality of functional groups. It serves to elasticize the curedadhesive bond and contains a plurality of polymerizable, olefinicallyunsaturated groups, for example vinyl or acrylic groups. It has a slightcrosslinking action, one crosslinkable group being contained in thepolymer (component C) for approximately 1000 molecular weight units.Readily compatible with the other components specified is, for example,a copolymer of butadiene/acrylonitrile having vinyl terminal groups,which may be present in the adhesive composition in a proportion of 10to 50 percent by weight. Further mechanical properties, such as, forexample, tensile strength and elasticity, are established by means ofthis component. The releasability of the cured adhesive bond is alsofavorably affected by the component C.

The photoinitiator system (component D) is so selected that thesensitivity maximum is between 300 and 400 nm, providing a sufficientlylong stability towards light of longer wavelength. That is to say, withdaylight or room illumination, a sufficiently long processing time ispossible in which the viscosity does not rise too sharply, so thatadhesive application, alignment and fixing of the semiconductor chipscan be carried out without difficulty. The selection of thephotoinitiator system is otherwise without further limitations, thesuitable systems being known in adequate number. The amount addeddepends on the desired curing rate or on the handling time requiredbefore curing.

Suitable tertiary amines are present for eliminating the oxygeninhibition.

The selection of the radical starter is also problem-free since, becauseof its low proportion in the adhesive composition, it hardly affects, ordoes not affect at all, the other properties of the latter. Well suitedare, for example, a number of peroxides, it being possible to use, forexample, the desired decomposition temperature or an adequate storagestability at room temperature and as low a curing temperature aspossible as selection criterion.

The adhesive according to the invention can be used, in particular, fora process for mounting semiconductor chips on a device, which processcomprises, according to the invention, the following process steps:

application of a UV-curable adhesive layer to the regions of a temporarycarrier provided as adhesion sites,

placing of the semiconductor chips to be mounted over the adhesion siteson the temporary carrier with the active side opposite the latersoldering site in a mirror-inverted arrangement (with respect to thelater mounting on the device),

determining the precise positions of the semiconductor chips on thetemporary carrier relative to a specified position and, if necessary,aligning the semiconductor chips in accordance with said specifiedpositions,

fixing the positions of the semiconductor chips by irradiating theadhesive layer with UV light,

curing the adhesive layer at elevated temperature,

soldering the rear side of the semiconductor chips glued to thetemporary carrier to the device in a soldering process, and

releasing the adhesive bond between the semiconductor chips and thetemporary carrier and dissolving all the adhesive residues on thesemiconductor chips by means of a solvent.

In the process, which is also according to the invention, the hithertounknown properties of the adhesive composition according to theinvention are exploited in an advantageous manner. This is a so-calledresoldering process in which a plurality of semiconductor chips arefirst glued to a temporary carrier in the desired, but mirror-invertedarrangement. Since the adhesive is UV-curing, the adhesive bonds of thechips to the temporary carrier can be examined and readjusted withrespect to a specified value before this position is fixed by means ofirradiation. The adhesive cures in the UV light within a few seconds sothat it is possible to place every chip individually, align it and fixit. The already mechanically fixed adhesive bond is exposed for a shorttime to an elevated temperature, in which process a complete curing ofthe adhesive layer takes place. The precise alignment or position of thesemiconductor chips on the temporary carrier remains intact in thisprocess.

In a soldering process, the semiconductor chips fixed on the temporarycarrier are now soldered to the final carrier, for example a device.Depending on the soldering process used, the cured adhesive layer isexposed under these conditions to a temperature of up to approximately215° C. Here again the precise alignment of the chips on the temporarycarrier remains intact, so that it is also achieved on the device whenthe solder hardens.

After the solder has cooled, the chips are fixed on the device, so thatthe temporary carrier can now be removed. The adhesive layer isdissolved with, in particular, organic solvents, it being possible tocarry out the removal completely and without leaving residues.

The precise position of the semiconductor chip on the temporary carriercan be determined by means of optical methods, in particular by means ofa microscope. In this process, the position of the semiconductor chipscan be determined relative to one another, or absolutely with respect tothe carrier, it being possible for measuring marks to be provided on thecarrier in the latter case. Advantageously, the optical measurement iscarried out at a wavelength which is outside the sensitivity range ofthe adhesive composition or its photoinitiator. If the sensitivity rangeis, as specified, between 300 and 400 nm, a color filter, for example ayellow filter, can be provided for the microscope light in order tofilter out components of the microscope light of shorter wavelength.

To fix the adhesive bond, any desired UV-light source whose light can bepresented to the adhesive bond, for example, via an optical waveguide isadequate. Thus, it is possible to fix individual adhesion sites withoutat the same time curing the adhesive already applied for thesemiconductor chips to be fixed later.

To finally mount the semiconductor chips fixed on the temporary carrieron the device, a vapor-phase soldering (vapor-phase process) may, forexample, be carried out. For this purpose, a solder paste is applied,for example by stamping or screen printing, to the planned solderingsite on the device. The semiconductor chips are now fixed with thetemporary carrier above the soldering site. The heat necessary for thesoldering process is supplied by means of inert organic vapors. In thisprocess, the entire arrangement is exposed to temperatures of up toapproximately 215° C. for several minutes. The adhesive bond of thesemiconductor chips to the temporary carrier withstands both the organicvapors and the elevated temperature without damage.

After cooling the solder, the adhesive bond is treated with an organicsolvent and completely dissolved thereby. The treatment with the solventcan be carried out by spraying, rinsing or immersion. For this purpose,strongly polar solvents or mixtures of a plurality of solvents aresuitable. Advantageously, a basic photoresist stripper can also be used.In this process, and also throughout the rest of the process, nocorrosion of the semiconductor components or of the metallic terminalpoints (bonding pads), which are composed, for example, of aluminum, isobserved.

In the case of less effective solvents, the removal of the adhesivelayer can be carried out in a bath and assisted by raising thetemperature or mechanically or with ultrasound.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail below by an exemplaryembodiment and four figures associated therewith. In this connection,the figures show different process stages in the mounting ofsemiconductor chips on a device using a temporary carrier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Exemplary Embodiment

A selected adhesive composition contains, for example, the followingcomponents, the amounts being specified in parts by mass (PM) and thetrade name and manufacturer being specified in brackets.

    ______________________________________                                        10.0 PM   Octyldecyl acrylate (ODA, Interez)                                  60.0 PM   Isobornyl acrylate (IBA, Alcolac)                                   10.0 PM   Ethoxylated trimethylolpropane                                                triacrylate (TMPEOTA, Interez)                                      40.0 PM   Vinyl-terminated                                                              butadiene-acrylonitrile-butadiene copolymer                                   (Hycar 1300 X23, Hycar)                                              6.0 PM   2-(Dimethylamino)ethyl benzoate (DMB,                                         Shell)                                                               2.0 PM   2-Methyl-1-(4-(methylthio)phenyl)-                                            2-morpholino-1-propa                                                          none (Irgacure 907, Ciba Geigy)                                      2.0 PM   Isopropylthioxanthone (ITX, Shell)                                   2.0 PM   1,1-Bis                                                                       (t-butylperoxy)-3,3,5-trimethylcyclohexane                                    (Luperox 231-50, supplied by Luperox)                                0.5 PM   Red pigment dispersion (LI-777, Loctite)                            ______________________________________                                    

The specified adhesive composition is a readily stampable material whichhas a viscosity of less than 600 mPas and which wets, for example,surfaces composed of glass or metal well and adheres well thereto evenafter curing.

It is now planned to mount a plurality of semiconductor chips in aspecified arrangement and with high precision (error tolerance, forexample, ±2 μm) on a device. For this purpose, the following procedureis adopted:

FIG. 1: an adhesive layer (21 to 26) of the material specified above isapplied at each of the specified adhesion sites to a temporary carrier 1which is composed, for example, of glass or metal. Suitable for thispurpose is, for example, a stamping process with which 5 μm thick layers21 to 26 are produced. The red-colored but transparent material isreadily processable in air and does not exhibit any marked increase inviscosity during the process. A first semiconductor chip 31 is nowplaced on the adhesive layer 21. The precise position of thesemiconductor chip 31 on the temporary carrier 1 is checked by means ofa microscope (not shown in the figure) and, if necessary, readjusted. Asan aid, markings can be applied for this purpose to the carrier 1 whichare detected, in the case of a temporary carrier composed of glass, intransmitted illumination and, in the case of a temporary carriercomposed of steel, by other suitable methods. The semiconductor chipsare normally placed and aligned in a fully automated manner. After thefirst semiconductor chip 31 has been aligned on the temporary carrier 1,the adhesive layer 21 is slightly cured by irradiation with UV light andthe adhesive bond fixed in this way. For this purpose, the regions ofthe adhesive layer 21 still accessible in the arrangement (1, 21, 31)can be exposed with the aid of an optical waveguide, the other adhesivelayers 22 to 26 present on the temporary carrier 1 not being affected bythe exposure and, consequently, not cured either.

FIG. 2 shows the temporary carrier 1, to which further semiconductorchips 32 to 36 have been successively applied, aligned and fixed in anappropriate manner on the corresponding adhesive layers 22 to 26. Therelative position of the semiconductor chips 31 to 36 corresponds to thearrangement mirror-inverted for the mounting on the device since thelater upper side of the semiconductor chips 31 to 36 are glued to thetemporary carrier. The adhesive layers, which produce the adhesivebonds, are cured to such an extent that the semiconductor chips areimmovably fixed. To completely cure the adhesive layers or the adhesivebonds, the arrangement shown in FIG. 2 is brought to an elevatedtemperature of approximately 120° C. for a short time. The peroxidescontained in the adhesive composition decompose in this process to formradicals and initiate the complete crosslinking and curing of theadhesive layer.

A solder paste is now applied, for example by screen printing, to thedevice in the regions provided for mounting. With the still free surfacedownwards, the semiconductor chips 31 to 36 mounted on the temporarycarrier 1 are placed, at the position provided, above the solder pasteand mechanically fixed in a suitable manner. In a vapor-phase solderingsystem, the arrangement comprising temporary carrier and component,together with semiconductor chips situated in between, is heated to upto 215° C. for a short time, in which process the solder present in thesolder paste melts. The adhesive bond of the semiconductor chips 31 to36 to the temporary carrier 1 withstands both the high temperatures andthe vapors of the organic solvent (for example Freon) used for the heattransfer without the semiconductor chips changing their position underthe conditions.

FIG. 3 shows the arrangement in which the semiconductor chips 31 to 36fixed or glued to the temporary carrier 1 are now securely mounted onthe device 5 by means of a solder joint, after the solder has cooled.The adhesive bonds of the semiconductor chips 31 to 36 to the temporarycarrier 1 are now released with a suitable solvent or a solvent mixture.For example, the adhesive bond may be sprayed with a photoresiststripper for this purpose. As a result of the adhesive layer swelling,the adhesive bond to the temporary carrier is first released and then,as a consequence of further treatment, all the adhesive residues on thesemiconductor chips 31 to 36 are removed.

FIG. 4 shows the arrangement after the removal of the temporary carrierand the adhesive residues. At the same time, the process is not limitedto a simple arrangement of the semiconductor chips as shown in thefigures, but on the contrary complex arrangements, even of differentsemiconductor chips, can also be mounted on a device. The precision inthe arrangement achieved in this process is not possible, or onlypossible with great effort, by direct soldering. Despite the detour viathe auxiliary fixing of the semiconductor chips on the temporary carrierthis is nevertheless a rapid process since the fixing of the adhesivebond requires only a few seconds. Placing and alignment of thesemiconductor chips on the temporary carrier take place at the usualspeed for highly automated processes.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

We claim:
 1. A process for mounting semiconductor chips on a device withthe aid of an adhesive composition, which comprises the followingprocess steps:applying a UV-curable adhesive layer to regions of atemporary carrier provided as adhesion sites, placing semiconductorchips to be mounted over the adhesion sites on the temporary carrierwith an active side opposite a later soldering site in a mirror-invertedarrangement with respect to a later mounting on the device, determiningprecise positions of the semiconductor chips on the temporary carrier(1) relative to a specified position and, if necessary, aligning thesemiconductor chips in accordance with said precise positions, fixingpositions of the semiconductor chips by irradiating the adhesive layerwith UV light, curing the adhesive layer at elevated temperature,soldering a rear side of the semiconductor chips glued to the temporarycarrier to the device in a soldering process, and releasing an adhesivebond between the semiconductor chips and the temporary carrier anddissolving substantially all adhesive residues on the semiconductorchips by a solvent.
 2. A process as claimed in claim 1, wherein saidstep of determining the precise positions of the semiconductor chips onthe carrier includes optical measuring by a microscope, and filteringout shorter-wave components from light used for this purpose by a colorfilter.
 3. A process as claimed in claim 1, wherein light used in saidstep of fixing is carried by an optical waveguide.
 4. A process asclaimed in claim 1, wherein said step of soldering comprises vapor-phasesoldering.
 5. A process as claimed in claim 1, wherein said solvent usedin said step of releasing comprises a photoresist stripper.
 6. A processas claimed in claim 1, wherein said UV-curable adhesive layer is of anadhesive composition comprising:A) at least one monofunctional(meth)acrylate of an aliphatic or cycloaliphatic alcohol containing 6-20carbon atoms, B) a polyfunctional reactive crosslinking agent based on alow-molecular-weight aliphatic polyhydric alcohol esterified with(meth)acrylic acid, C) oligomers or polymers containing olefinicallyunsaturated groups, which can be copolymerized with acrylates and have aslight crosslinking action, for the purpose of flexibilization, D) aphotoinitiator system for UV light, and E) a thermally activable radicalstarter, from which adhesive composition an adhesive bond can beproduced which is resistant to a soldering process and which can also bedissolved with a solvent after the soldering process to leave noresidue.